With the significant increases in energy costs which have taken place over the last few years, it has become essential, particularly for large installations, to find ways to reduce the energy consumption of cooling systems to make them more economical to operate. In recent years many steps have been taken to improve the insulation factors of buildings and to minimize the leakage of air from the buildings in order to reduce the costs of cooling such buildings.
The primary focus for reducing the costs of cooling buildings, however, must be concentrated on the cooling apparatus itself. It is possible to reduce the amount of energy consumed by such apparatus, even by a small percentage, significant savings in operating costs over a period of time can be realized. Many installations currently employ a closed loop cooling system in which the coolant for the system passes from a load (where it picks up heat) through an evaporator in a chiller and back to the load. The chiller includes a compressor and a condenser and in many installations the coolant for the condenser is water which is circulated in an open loop from the condenser to spray nozzles in a cooling tower. The water collected at the base of the cooling tower then is supplied back to the condenser to permit heat exchange between the condenser in the open loop, the compressor, and the evaporator in the closed loop. Thus, in the chiller, heat is given up by the coolant fluid in the closed loop through the evaporator to the water in the open loop in the condenser and the cycle continuously repeats.
It has been discovered that under some ambient conditions, "free" cooling can be realized without the expensive operation of the chiller or primary heat exchange unit. One approach for accomplishing this is disclosed in the patent to Newton U.S. Pat. No. 2,352,282. Newton discloses a system which has two heat exchange coils located in series in the air duct of the air conditioning system. One of these coils is supplied with a low cost coolant, such as a supply of city water. Upon initial demand for cooling, the water cooling coil is the only one used. As demand for cooling increases, a conventional mechanical refrigeration coil is used to remove heat from the air passing through the duct; and the water cooling coil is turned off. As still further increases in demand for cooling exist, the water cooling coil is turned back on to operate in conjunction with the mechanical refrigeration coil. The two cooling loops are independent of one another except for the heat exchange units located in the air duct. Due to the fact that there are two coils located in series in the air duct, increased static pressure loss with associated higher energy costs for moving a unit of air exists in this system.
A different attempt in a commercial system for supplementing the cooling which is attainable from a primary heat exchange unit is disclosed in the patents to Morse U.S. Pat. No. 4,144,723 and Schmitt U.S. Pat. No. 4,315,404. These systems both require storage reservoirs. Whenever the primary heat exchange device cannot provide a cold enough fluid temperature, the fluid is supplied through an after cooler to lower the fluid temperature before it is supplied to the load. The systems are used on days of peak ambient dry bulb temperature; and in order to be effective, require a substantial storage system or storage capability for the coolant used in the after cooler or secondary heat exchange device.
Some installations also presently use an either/or arrangement of different heat exchangers (Iversen U.S. Pat. No. 3,995,443). A primary mechanical refrigeration unit or heat exchange unit is employed to handle the cooling in the system whenever relatively high ambient temperatures exist. A secondary system is used whenever the ambient dry and/or wet bulb temperatures are such that an evaporative cooling system is capable of handling the load. In such systems, either one or the other of these two heat exchange systems or devices is used. Most frequently the load is shifted from one to the other manually or in some cases by using an automotive control system responsive to preestablished set point conditions. Thus, the secondary system is used when it is considered capable of handling the load. The load then is shifted back to the primary mechanical unit when the secondary system is no longer considered capable of handling the load. The conditions for using the secondary heat exchange system are that the ambient dry and/or wet bulb temperatures must be low enough for the secondary system to produce a fluid temperature below that which is required leaving the primary heat exchange apparatus at the time the secondary apparatus is used. Because of the manner in which these "either/or" systems function, they are relatively limited in the number of hours or days during which the more economical secondary evaporative cooler heat exchange unit is actually utilized.
It is desirable to provide a system which automatically provides load shaving for the primary heat exchange apparatus in a cooling system, and which does this in a manner to reduce the energy requirements of the overall heat exchange system.